Telephones and other types of data terminal equipment (DTE) are routinely used for voice data and other forms of telecommunication. Examples of these include voice communication devices or telephones operating through a private branch exchange (PBX) or key telephone system. PBX-type and key telephone system-type telephones typically require the application of external electrical power in order to operate. The power is used to energize circuits within the telephone circuitry, such as processing displays and the like.
There are essentially two practical methods available for powering such telephones. The first method provides power directly to the telephone. This may be accomplished by a wall-mounted transformer providing a direct current (DC) or alternating current (AC) low voltage power supply to the telephone from the electric power grid. Such direct power methods have a number of drawbacks. Apart from continuous occupation of a potentially scarce electrical outlet, during a power grid disruption, the power to the telephone may be disconnected rendering the telephone inoperative and effectively precluding emergency telephone usage during the black out.
The second method for providing telephone power involves supplying so-called “phantom power” by coupling a power signal (typically up to about 48 volts DC) to the telephone over twisted pair lines as taught, for example, in U.S. Pat. No. 6,115,468 entitled “Power Feed for Ethernet Telephones via Ethernet Link”. In accordance with a specific embodiment of the aforementioned patent, as illustrated in FIG. 1 hereof, a system 10 including a plurality of data ports 12, 14, and 16 within a communication device such as a switch 18 is provided to transmit data on lines 20 and 22, 24 and 26, and 28 and 30 to and from first, second, and third load devices 32, 34, and 36, respectively. Load devices 32, 34, and 36 may be associated with Ethernet telephones and/or other Ethernet devices 38, 40, and 42 respectively, requiring power to be transmitted to them in addition to data over Ethernet twisted pair lines.
Turning to the first port 12 in FIG. 1 as an example, data on lines 22 and 20 are provided to transmitter 44 and from receiver 46, respectively. Transmitter 44 drives data traffic along pair 48, while receiver 46 receives data from pair 50. First transformer 52 includes a first primary 54 connected to pair 48, and a second primary 56 connected to pair 50.
A similar arrangement exists at first load 32. Second transformer 58 includes a first secondary 60 and a second secondary 62. These are respectively connected to pairs 64 and 66 that are, respectively, connected through receiver 68 and transmitter 70 to load device 32. Ethernet link 72 couples first and second secondaries 74 and 76 of first transformer 52 to first and second primaries 78 and 80 of second transformer 58. Ethernet link 72 preferably comprises a pair of twisted pair conductors 82a and 82b, wherein twisted pair 82a connects first secondary 74 to first primary 78 and twisted pair 82b connects second secondary 76 to second primary 80.
Power coupling may be provided as follows: Secondaries 74 and 76 of first transformer 52 each include a center tap 84 and 86 respectively. Center tap 84 connects to the positive lead 88 of a power supply 90, and center tap 86 connects to the negative lead 92 of power supply 90, which may, preferably, include an uninterruptible power supply (UPS). DC filtering components 94a, 94b, and 94c may be provided as well known to those of ordinary skill in the art. Now the entire loop of twisted pair 82a is at the potential of positive lead 88 while the entire loop of twisted pair 82b is at the potential of negative lead 92 without any impact on data transmission. A power processor 96 may preferably be attached to center taps 98 and 100 of first and second primaries 80 and 82 of second transformer 58, respectively, so as to provide a DC power source to the power processor 96.
The power processor 96, shown in more detail in FIG. 2, conventionally includes a filter 102, a rectifier 104, a filter capacitor 106, and a DC—DC converter 108. Other similar arrangements are also well known to those of ordinary skill in the art. The power processor 96 may perform DC—DC power conversion and filtering required, as well as provide power over leads 110 and 112 to load 32, which may be an Ethernet telephone or other device.
Returning to FIG. 1, the communication system 10 may be divided into a switch 18 and DTE devices 38, 40 and 42. The switch 18 may be found within a telephone equipment room, for example. Cables may then be routed from the load devices 32, 34, and 36 to the switch 18 for connecting to conventional jacks or connectors of the switch (not shown) such as the well-known type RJ-45 connectors (as used herein the term RJ-45 type connector is intended to include both male (plugs) and female (sockets) variants thereof). Because the switch 18 supplies a DC bias to the center taps 84 and 86, and because the switch 18 may conceivably be used with legacy DTE potentially not compatible with the application of such a voltage, it would be desirable to provide a mechanism for detecting which ports of such a switch 18, should and should not have phantom power applied. Phantom power should then be supplied to a DTE if the switch establishes that the DTE is a compatible IP telephone or other compatible powerable network device. However, a priori determination of this condition is not presently available, and thus a requirement for power at the DTE cannot presently be determined. Accordingly, there exists a need for an efficient, simple, and inexpensive method and apparatus for detection of a compatible IP device that should be provided with phantom power.